1. Field of the Invention
The present invention relates to a technique for performing screen processing.
2. Description of the Related Art
In the field of printing, gradation is expressed by performing screen processing on input data and controlling on and off of dots according to the screen processing result.
When screen processing is performed on the input data, the input data is quantized using a threshold value group (hereinbelow referred to as a screen) in which large and small threshold values are periodically disposed. More specifically, the input data of a target pixel is compared with a screen threshold value corresponding to the target pixel. If the input data is greater, the dot becomes on, and if the input data is smaller, the dot becomes off. For example, there is an amplitude modulation (AM) screen processing technique in which the threshold values are periodically disposed according to setline per inch and inclination angle (i.e., a screen angle), which are properties of the screen used.
However, if the input image includes thin lines or an edge, the image and the screen interfere with each other in a case where the direction of the thin line or the edge is close to the inclination angle. As a result, the lines may be broken, the jaggy may be generated at the edge, or moiré may be generated. Japanese Patent Application Laid-Open No. 2004-040499 discusses a technique which extracts a contour region in graphic data and switches the screen.
Further, Japanese Patent Application Laid-Open No. 2010-074627 discusses a technique which combines a plurality of screens whose screen angles are the same and whose numbers of lines are integral multiples, according to a characteristics amount of the edge or the thin lines in the input image.
Furthermore, Japanese Patent Application Laid-Open No. 2006-174068 discusses a technique which increases reproducibility of the thin lines or the edge without determining and detecting the thin lines or the edge. More specifically, a plurality of screens of different screen angles is prepared, and screen-processed data is generated for each pixel block of a predetermined size. An error between each of the screen-processed data and the input image is then evaluated for each pixel block, and screen processing in which the error is smallest is selected.
However, according to the above-described techniques, there are problems as follows. It is necessary in methods discussed in Japanese Patent Application Laid-Open No. 2004-040499 and Japanese Patent Application Laid-Open No. 2010-074627 to determine whether the image includes the thin lines or the edge. If there is an error in such determination, appropriate processing cannot be performed. Further, since the screen angles of the plurality of screens are the same in the method discussed in Japanese Patent Application Laid-Open No. 2010-074627, the thin lines may be broken and jaggy may be generated at the edge.
Furthermore, according to the technique discussed in Japanese Patent Application Laid-Open No. 2006-174068, screen processing is evaluated using a density error in a predetermined block. As a result, breaking of the thin lines, a jaggy edge, and a moiré generated due to interference cannot be correctly evaluated, so that screen processing having high reproducibility cannot be performed.